The amount of water consumption of natural plant is a part of its amount of water requirement, and also the basis to determine its amount of water requirement. Ejina County lying in the lower reaches of Heihe River basins, belongs to an exceedingly arid region, and only on the bank of Heihe River, there are banded oases nourished by groundwater supplied by the river water in the upper and middle reaches, the left region all is covered by desert. In Ejina County, the main landscape is natural oasis. Alhagi sparsifalia is a familiar kind of perennial natural undershrub in the natural oasis, it has the common character of natural plants in Ejina County, its underground biomass is more than its overground biomass and its lateral roots is very abundant. So to make clear the rule of water consumption and to determine the ecological amount of water consumption of Alhagi sparsifalia will provide scientific basis for determining that of other natural plants in Ejina County. At the same time, this will provide a scientific basis for maintaining present ecological environment of Ejina County. The ecological amount of water consumption of Alhagi sparsifalia is successfully calculated by potential evapotranspiration multiplying a reduction factor, and the reduction factor is determined by soil moisture volume percentage and roots efficacious length. It is obtained that the annual amount of water consumption in natural Alhagi sparsifalia woods in Ejina County is 132.002 mm, mainly coming from the phreatic water evaporation and the soil moisture storage.